In known radio systems, the reception moment of a signal arriving from a transmitter at a receiver helps to determine the location of a subscriber terminal in a radio system. In a prior art method, reception moments of signals transmitted by transmitters, e.g. base stations, are measured with respect to each other. This measured time difference of the signals is also called an Observed Time Difference (OTD).
Generally, transmitters are synchronised in such a way that lower levels of a radio system are synchronised by a synchronising signal coming from an upper level of the radio system. If base stations transmit a signal synchronously, the observed time difference is obtained solely by examining the distances of the base stations. The observed time difference of two synchronously functioning transmitters can thus be calculated by means of a so-called Geometrical Time Difference (GTD). If the locations of the transmitters are fixed and known in advance, the actual location of the subscriber terminal in the radio system can easily be calculated by means of propagation times of the signals. In practice, however, base stations functioning as transmitters do not totally synchronise with each other, and so errors occur when determining locations of subscriber terminals.
In known radio systems, the Real Time Difference of a radio transmitter, i.e. the time difference between transmission times of two transmitters, can be determined by the GTD calculated on the basis of the locations of transmitters and receivers, and by the OTD measured by receivers. The geometrical time difference between two signals is determined by means of the difference in the distance travelled by both signals, and by means of the propagation velocity of the signal. In known radio systems, locating a subscriber terminal is based on the signals transmitted by transmitters and received by receivers. On the basis of signals received by a receiver, information is obtained on the mutual synchronisation of the transmitters. Practically, the known radio systems comprise various receivers, and it is thus complicated to process time difference information based on the signals received by the receivers in such a way that the real time difference and the synchronisation between the various transmitters could easily be detected.